The present invention relates to a portable generator which generates an AC voltage of 100 V or the like by being turned by an engine.
Today, small generators driven by a gasoline engine or a diesel engine, permitting conveyance to where they are needed and capable of developing a per-unit output of several kilowatts to ten-odd kW, have come into extensive use.
Such portable generators permitting ready conveyance include generators enabled, by keeping the frequency of engine revolutions constant, to develop a single-phase AC voltage of around 100 V in average output voltage at a frequency of 50 or 60 Hz.
However, more recently, there have been proposed systems whereby the output of an engine-driven AC generator is once rectified into a DC voltage and this DC voltage is further converted with an inverter into an output voltage having a constant frequency of 50 Hz or 60 Hz (e.g. JP 63-114527 A and JP 63-302724 A).
An engine-driven small portable generator capable of developing an output of several kilowatts to ten-odd kW is not only carried to where they are needed and used for power generation always in a movable state but also may be semi-permanently installed in a fixed position where it is required to operate continually for some time.
This inverter-equipped portable generator, as shown in FIG. 10, has an engine-driven AC generator 50, a DC-voltage-generating circuit 110 using rectifier diodes 115 and thyristors 111, a DC-power-source unit 120 using a large-capacitance capacitor 121 consisting of a required number of capacitors connected in parallel, an inverter circuit 130 using a power transistor, and a low pass filter 140 using a coil and a capacitor.
It further has, as control circuits for driving and controlling such power circuits as these DC-voltage-generating circuit 110 and inverter circuit 130, a PWM-signal-generating circuit 250, a voltage-limiting circuit 240, an overload-detecting circuit 260 and an inverter-drive circuit 255. This portable generator 100 also has, as power-supply units for driving these control circuits, a smoothing circuit 210 and a constant-voltage circuit 235.
Many of the AC generator 50 in use whose rotor is turned by such an engine has a three-phase output coil 51 and a single-phase output coil 55. In many cases, the three-phase output coil 51 can develop a maximum output of tens of amperes at hundreds of volts, while the single-phase output coil 55 can develop an output of tens of amperes at tens of volts.
The DC-voltage-generating circuit 110 to which the output terminal of this three-phase output coil 51 is connected is configured of a rectifier bridge circuit using three rectifier diodes 115 and three thyristors 111. The both output terminals of this rectifier bridge circuit is connect to both ends of the main smoothing capacitor 121, which uses the DC-power-source unit 120, to charge the capacitor 121.
Incidentally, the gate terminal of each thyristor 111 in the DC-voltage-generating circuit 110 is connected to the voltage-limiting circuit 240 to control the continuity angle of each thyristor 111, and the voltages at both ends of the main smoothing capacitor 121, which uses the DC-power-source unit 120, are thereby regulated.
The inverter circuit 130 is configured of a bridge circuit using four power transistors. In this inverter circuit 130, a first transistor 131 and a third transistor 133, arranged in series, are connected to the DC-power-source unit 120, and a second transistor 132 and a fourth transistor 134, arranged in series, are connected to the DC-power-source unit 120. The midpoint between the first transistor 131 and the third transistor 133 is connected to a first output terminal 151 via the lowpass filter 140, and the midpoint between the second transistor 132 and the fourth transistor 134 is connected to a second output terminal 152 via the low pass filter 140. Further the base of the first transistor 131 and the base of the fourth transistor 134 are commonly connected to the inverter-drive circuit 255, and the base of the second transistor 132 and the base of the third transistor 133 are commonly connected to an inverter-drive circuit 255.
A first PWM signal supplied from this inverter-drive circuit 255 to the first transistor 131 and the fourth transistor 134 and a second PWM signal supplied from this inverter-drive circuit 255 to the second transistor 132 and the third transistor 133 are high-frequency pulse signals of several kHz or more. The pulse width of each pulse signal is successively varied between 50 Hz and 60 Hz, and the varying quantity of the pulse width is successively increased or decreased in a sine-wave shape.
Further, the first PWM signal and the second PWM signal are reverse in phase to each other. For this reason, continuity is established between the first transistor 131 and the fourth transistor 134 by the first PWM signal, while discontinuity is ensured between the second transistor 132 and the third transistor 133 by the second PWM signal, and when the midpoint between the first transistor 131 and the third transistor 133 has a voltage VD, which is the voltage of the DC-power-source unit 120, the midpoint between the second transistor 132 and the fourth transistor 134 is at 0 V. When continuity is established between the second transistor 132 and the third transistor 133 by the second PWM signal, the first PWM signal ensures discontinuity between the first transistor 131 and the fourth transistor 134, sets the midpoint between the first transistor 131 and the third transistor 133 to 0 V, and the midpoint between the second transistor 132 and the fourth transistor 134 then to the voltage VD of the DC-power-source unit 120.
This midpoint potential between the first transistor 131 and the third transistor 133 changes over at high speed between 0 V and the voltage VD of the DC-power-source unit 120 as shown in FIG. 11A, and the duration of the DC source voltage VD successively varies. Also, the midpoint potential between the second transistor 132 and the fourth transistor 134 also changes over at high speed between 0 V and the voltage VD of the DC-power-source unit 120 as shown in FIG. 11B, and the duration of the DC source voltage VD successively varies.
As a result, a first output voltage and a second output voltage having passed the low pass filter 140 using a coil and a capacitor are cleared of harmonic contents and are turned into sine-wave voltages of 50 Hz or 60 Hz as shown in FIG. 11. Then, the voltage of the first output terminal 151 and the voltage of the second output terminal 152 are generated as AC output voltages of 50 Hz or 60 Hz averaging 100 V, with their peak level and bottom level staggered by a half period.
On the other hand, the single-phase output coil 55 of the AC generator 50 is connected to the smoothing circuit 210 in the control-power-supply circuit as shown in FIG. 10.
This smoothing circuit 210 is configured of a rectifier diode 211 and a smoothing capacitor 215. The rectifier diode 211 is inserted between the output terminal of the single-phase output coil 55 and the smoothing capacitor 215, and the smoothing capacitor 215 is charged with the output voltage of the single-phase output coil 55 to form a DC voltage.
Incidentally, the number of the rectifier diode 211 is not limited to one as shown in FIG. 10, but sometimes four rectifier diodes are used as an all-wave rectifier bridge to charge a smoothing capacitor.
Then, the output terminal of the smoothing circuit 210 is connected to the constant-voltage circuit 235, and this constant-voltage circuit 235 generates a prescribed voltage for driving control circuits.
Further, the terminal on the xe2x88x92 side of this constant-voltage circuit 235 is connected to the + side of the DC-power-source unit 120, and the terminal on the + side of the constant-voltage circuit 235 is connected to the voltage-limiting circuit 240, the PWM-signal-generating circuit 250 and an inverter-drive circuit 255.
This voltage-limiting circuit 240 is configured of resistors and comparators. The first reference-voltage resistor 245 and the second reference-voltage resistor 246, arranged in series, are inserted between the + side terminal of the constant-voltage circuit 235 and the + side terminal of the DC-power-source unit 120, and the midpoint between the first reference-voltage resistor 245 and the second reference-voltage resistor 246 is connected to the reference input terminal of a comparator 243. The first voltage-dividing resistor 248 and the second voltage-dividing resistor 249, arranged in series, are inserted between the + side terminal of the constant-voltage circuit 235 and the xe2x88x92 side terminal of the DC-power-source unit 120, and the midpoint between the first voltage-dividing resistor 248 and the second voltage-dividing resistor 249 is connected to the comparing input terminal of the comparator 243.
Further, the output terminal of the comparator 243 is connected to the + side terminal of the constant-voltage circuit 235 via a control resistor 241 as well as to the gate terminal of each thyristor 111 in the DC-voltage-generating circuit 110. In connecting the output terminal of the comparator 243 to the gate terminal of each thyristor 111, it is connected via a protective resistor 117.
Therefore, this voltage-limiting circuit 240 can form a fixed reference voltage by causing the first reference-voltage resistor 245 and the second reference-voltage resistor 246 to divide a fixed voltage generated by the constant-voltage circuit 235 of the control power supply circuit. Further, this reference voltage fixed all the time can be entered into the reference input terminal of the comparator 243.
Also, a voltage resulting from the addition of the output voltage of the DC-power-source unit 120 and a fixed voltage generated by the constant-voltage circuit 235 is divided by the first voltage-dividing resistor 248 and the second voltage-dividing resistor 249 to form a detection voltage, and this detection voltage can be entered into the comparing input terminal of the comparator 243.
As a result, the detection voltage entered into the comparing input terminal varies with the voltage variations of the DC-power-source unit 120 and, if this detection voltage is lower than the reference voltage generated by the first reference-voltage resistor 245 and the second reference-voltage resistor 246, the output of the comparator 243 will be a + potential.
Therefore, the gate potentials of the thyristors 111 can be kept higher than the cathode potentials of the thyristors 111, and a gate current can be supplied to each thyristor 111 via the control resistor 241 to establish continuity of each thyristor 111. For this reason, when the output voltage of the three-phase output coil 51 becomes higher than the voltage of the DC-power-source unit 120, power is supplied to the DC-power-source unit 120 to raise the voltage of the DC-power-source unit 120.
Further, when the voltage of the DC-power-source unit 120 rises and the detection voltage entered into the comparator 243 becomes equal to the reference voltage, the output of the comparator 243 becomes 0. Therefore the gate potential of each thyristor 111 becomes equal to the cathode potential to place each thyristor 111 in a state of discontinuity.
Thus, when the voltage generated by the DC-power-source unit 120 is made lower than a fixed voltage by the voltage-limiting circuit 240, the AC generator 50 performs charging and, when the charged voltage reaches the fixed voltage, stops charging. As a result, it is possible to keep the output voltage of the DC-power-source unit 120 somewhere between 170 V and 200 V to keep the fixed voltage VD set by the voltage-limiting circuit 240 all the time.
Then, the inverter circuit 130 varies the potentials of the first output terminal 151 and the second output terminal 152 in a fixed period of 50 Hz or 60 Hz, and a single-phase AC voltage is supplied with the maximum potential difference between the voltage of the first output terminal 151 and the voltage of the second output terminal 152 being 141 V and the average voltage being 100 V.
The PWM-signal-generating circuit 250 which generates a PWM control signal for controlling this inverter circuit 130 generates the PWM control signal from a reference sine-wave such as 50 Hz, 60 Hz or the like and a triangular wave and supplies it to the inverter-drive circuit 255.
The reference sine-wave of the PWM-signal-generating circuit 250 is generated in accordance with a prescribed frequency, such as 50 Hz or 60 Hz, which is the frequency of the voltage supplied from the output terminal. This PWM-signal-generating circuit 250 regulates the ratio between the voltage of the reference sine-wave and the voltage of the triangular wave, and determines the frequency, pulse width and the quantity of width variation of the pulse signal, which is used as the PWM control signal according to the output voltage VD of the DC-power-source unit 120 entered into the inverter circuit 130 and the characteristics of the inverter circuit 130 and the low pass filter 140.
Further this portable generator 100 is provided with the overload-detecting circuit 260, wherein a detecting resistor 261 is inserted between the DC-power-source unit 120 and the inverter circuit 130.
This overload-detecting circuit 260 is configured of the detecting resistor 261 and an arithmetic-circuit unit 265. When having detected an amperage surpassing the rated amperage, this overload-detecting circuit 260 supplies a stop signal to the inverter-drive circuit 255 according to the extent of surpassing the rating with the time factor also taken into account.
This arithmetic-circuit unit 265 uses various circuits having comparators, capacitors and resistors. It takes into account the characteristics of the elements constituting the power circuit and, in many cases, immediately issues a stop signal when a current of double the rated amperage flows to stop the output of the inverter-drive circuit 255 from supplying the first PWM signal and the second PWM signal. The arithmetic-circuit unit 265 is designed to issue a stop signal to the inverter-drive circuit 255when it has detected a current slightly surpassing the rated amperage and this current flow has continued for several seconds to several minutes.
In this portable generator 100, in which a three-phase AC voltage once rectified by the DC-voltage-generating circuit 110 and the DC voltage generated by the DC-power-source unit 120 is again converted into an AC voltage by the inverter circuit 130 can generate an AC output voltage whose frequency and voltage are stable all the time while forming a power matching the load by varying the revolutions of the AC generator 50, i.e. revolutions of the engine.
Therefore, this portable generator 100 can adjust the engine revolutions to load variations, increase the revolutions when the load is heavy, and decrease the revolutions when the load is light, thereby making it sufficient for the engine to generate the quantity of energy that the load requires, accordingly can readily adjust the output to the load level, and therefore operate efficiently.
When it becomes overloaded beyond the rated output, the generator can stop the inverter circuit 130 from operating promptly or after the lapse of a prescribed length of time, bring down the output voltage to 0, and operate various electric devices with which the generator is loaded within an extent of several kilowatts, which is its rated output, while maintaining the overall safety of the circuitry.
Thus, the engine-driven portable generator 100 using the inverter circuit 130, for its capability to supply single-phase AC power of 100 V as does a commercial power source, has come to be used for supplying power to various electrical devices in general.
However, the above-described engine-driven portable generator (100) using an inverter may apply a DC voltage to electrical devices connected to it and thereby damage the devices because a DC voltage remains at its output terminals (151, 152) even after the engine is stopped.
Moreover, when anything of low impedance or low resistance is inadvertently brought into contact with the output terminals when the engine is not running, short circuiting may occur.
The present invention is intended to obviate these disadvantages, and to ensure that any residual voltage at two output terminals (151, 152) is eliminated when a portable generator (100) is stopped from providing an output.
Thus, the invention provides a portable generator (100) turning an AC generator (50) by an engine to form an AC voltage, once rectifying the AC voltage into a DC voltage, converting the DC voltage into a fixed single-phase AC voltage of a prescribed frequency by an inverter circuit (130), and outputting the single-phase AC voltage, via a low pass filter (140), through output terminals (151, 152); which includes an output-stop-control unit (443) for stopping the inverter circuit (130) at the time when the outputted single-phase AC voltage, by an operating switch (305) being turned off, drops to 0 V.
Thus, because the operation of the inverter circuit (130) is stopped at the time when the voltage between the output terminals drops to 0 V, the charge accumulated in the capacitor of a low pass filter (140) provided following the inverter circuit (130) can be reduced substantially to 0, and the voltage generating at the output terminals (151, 152) when the portable generator (100) stops can be eliminated.